SITE INFORMATION AND CORRESPONDENCE

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Wendy E. Cohen -3= • 4 October 1996 Sixty to 100 percent of the City of Tracy's water is supplied from the Delta-Mendota Canal. When the canal cannot supply 100 percent of the city's demand, water from the city's 10 ground water wells is used as a supplement. These wells contribute equally to provide up to 40 percent of the city's water demand. The city's closest well, the South Area Well, is approximately 50 feet from the GP's stormwater pond. This well does not contain chloroform. Ground Water Remediation The first recovery well (RW1) was installed in May 1985. Extraction from this well continued until installation of the second recovery well (RW2) at which time extraction from RWl was discontinued. RW2 was installed because RWl was not completely effective in removing chloroform from the upper 5 to 10 feet of the shallow ground water. RW1 is screened between 56.7 and 75.5 feet bgs and RW2 is screened between 40 and 60 feet bgs. The first sample from RWl collected on 24 June 1985 had no detectable chloroform, but chloroform concentrations increased to a maximum of 1,600 µg/l on 20 August 1985. Since August 1985, chloroform in RWl decreased to 27 Mcg/l on-16 January 1990. Extraction from RW2 began in January 1990, with the first sample collected on 1 March 1990 containing 17 µg/1 chloroform. The last sample from RW2, collected on 1 September 1991 had 6.3 µg/1 chloroform. A letter from GP on 3 September 1991 requested Board approval for ground water cleanup activities to cease. In a 6 December 1991 letter, staff did not approve the request due to concerns over the appropriate cleanup goals for chloroform. A cleanup goal of 0.2µg/1 chloroform set by staff on 14 February 1985 was updated to 5 µg/I in December 1991. A 4 February 1992 monitoring report from GP reported that RW2 and the air stripper system were not monitored because the system was down for repair. GP has not yet repaired the system due to the "pending" negotiations with the Board. Source area wells (BC2, BC3, BC4, BCS, BC7, RW2, and RWl) have shown significant decreases in chloroform levels. A maximum chloroform concentration of 23,000 µg/I was detected in BC2 on 27 February 1986. Monitoring wells BC2, BC3, BC4, and BC5 have been intermittently, or consistently, dry since between 1990 and 1994. The last samples in these source area monitoring wells ranged from 31 to 180 µg/1 chloroform. Monitoring well BC7 has not been dry. Chloroform in this well has been variable, with the highest concentration of 3,200 µg/1 in November 1985. Since then, chloroform has decreased to 2.8 µg/1 in January 1996. Since July 1993, chloroform in this well has varied between 1.2 µg/1 and 44 µg/1. Three wells are located on the perimeter of the chloroform plume, with BC14 on the downgradient edge. Chloroform concentrations in this well have ranged from nondetect to 75 µg/I (11 July 1989). Since November 1993, chloroform has ranged from 21 to 31 µg/1 in this well. The other two perimeter wells, BC15 and BC16, have had chloroform concentrations ranging from 1.4 to 25 µ9/1 and nondetect to 28 jLg/1, respectively. The maximum concentration in both of these wells occurred in January 1988. Since then chloroform in both wells has generally decreased. The latest results, from 23 January 1996, had 3.2 jLg/1 and nondetectable chloroform concentrations in BC15 and BC16, respectively.